1. Technical Field of the Invention
The present invention relates to a voltage-controlled oscillator and an electronic device using the same. More particularly, the present invention relates to a voltage-controlled oscillator incorporating a plurality of SAW resonators, one of the plurality of SAW resonators is selected in accordance with a control signal, and the oscillation frequency is varied in accordance with a control voltage, and to an electronic device using the voltage-controlled oscillator.
2. Related Art
In telecommunications devices such as cellular phones, communication data is transmitted and received based on the clock signal from an oscillator. Communication networks have advanced toward broadband services, the demand of the market has moved to a high-frequency band exceeding 400 MHz, and in this high-frequency band, data is transmitted and received. In electronic devices, including communication devices in recent years, there has been a demand for high-frequency oscillators for providing higher communication speed with good frequency stability at a high-frequency band, temperature be compensated for in the practical temperature range of the communication device, and jitter of the clock signal output from the high-frequency oscillator be small. In particular, in a high-speed network market, such as Ethernet (registered trademark) using a gigabit band and a fiber channel, which have shown rapid growth in recent years, in order to prevent communication error from occurring which results from jitter of the high-frequency oscillator, there has been a demand for a highly stabilized high-frequency oscillator in which jitter is very small.
At present, there is an optical fiber communication system of 10 Gbit/s which has been put to practical use, for example, the SONET (Synchronous Optical NETwork) method in the United States. In this optical fiber communication system of 10 Gbit/s, several frequencies are used due to the difference of the data structure to be transmitted and received, four nearby frequencies exist in the band of 600 MHz, and high-frequency oscillators corresponding to respective frequencies are necessary. The difference of these frequencies is chiefly due to the difference of the number of bits of transmission data added by the error correction function for identifying the presence or absence of the error data generated on the transmission line and the difference in the structure of the transmission and reception data according to the method. Specifically, there are frequencies of 622.08 MHz, 644.53125 MHz, 666.51429 MHz, and 669.32658 MHz.
As conventional voltage-controlled oscillators, those shown in FIGS. 20 and 21 are known.
FIG. 20 is a block diagram of a voltage-controlled oscillator (VCO) 1G which consists of a parallel resonance circuit using an LC. This voltage-controlled oscillator (VCO) 1G comprises a Colpitts-type oscillation circuit 80 having a parallel resonance circuit 83 which is a feedback section, a multiplier circuit 81, and a differential conversion circuit 82. The Colpitts-type oscillation circuit 80 is composed of an NPN-type bipolar transistor (not shown), etc., and the parallel resonance circuit 83 is composed of an inductor L, a capacitor C, and a variable capacitance diode Cv. By controlling the variable capacitance value of the variable capacitance diode Cv in accordance with a control voltage Vc, the oscillation frequency varies. The oscillation output from the Colpitts-type oscillation circuit 80 is further multiplied to a desired frequency, for example, 622.08 MHz at the multiplier circuit 81, and the oscillation output is output as a clock signal at the differential conversion circuit 82 from which a differential signal can be extracted from one input signal.
FIG. 21 is a block diagram showing the configuration of a conventional voltage-controlled crystal oscillator 1H using an AT-cut crystal vibrator. This voltage-controlled crystal oscillator (VCXO) 1H comprises an oscillation section 85 formed of an AT-cut crystal vibrator which vibrates at several tens of MHz, the voltage-controlled phase-shift circuit 84, and the Colpitts-type oscillation circuit 80; the multiplier circuit 81; and the differential conversion circuit 82. In the oscillation section 85, the external control voltage Vc is input into the voltage-controlled phase-shift circuit 84 so that the oscillation frequency can be varied within a fixed range.
In a conventional voltage-controlled crystal oscillator (VCXO), the oscillation section 85 can also be formed so as to be replaced with a multi-frequency switching-type oscillator for selecting a signal from a plurality of piezoelectric vibrators (crystal vibrators) by using the switching control circuit shown in, for example, Japanese Unexamined Patent Application Publication No. 7-297640 (Paragraph 0013, FIG. 1).
As a conventional voltage-controlled oscillator, for example, there is a voltage-controlled oscillator in which, as shown in Japanese Unexamined Patent Application Publication No. 8-213838 (Paragraph 0006, FIG. 1), a plurality of SAW resonators are switched by a switch so as to achieve a broader band with low noise.
The above-described voltage-controlled high-frequency oscillator has problems such as those described below.
The voltage-controlled oscillator (VCO) 1G shown in FIG. 20 has problems in that, because the Q value of a parallel resonance circuit using a coil and a capacitor is very small, a lot of jitter occurs, and thus system specification concerning the jitter of the SONET method cannot be satisfied.
On the other hand, when the voltage-controlled crystal oscillator (VCXO) 1H using an AT-cut crystal vibrator having a high Q value, shown in FIG. 21, is used, there are problems in that the change in the frequency of the AT-cut crystal vibrator remains about several hundreds of KHz, and it is not possible for a high-speed optical communication system to be used to deal with multi-rate.
Next, in the AT-cut crystal vibrator described in FIG. 21 and JP 7-297640, if, in addition to main vibration, sub-vibration exists nearby, when the temperature condition changes, the sub-vibration is further brought closer to the main vibration and is likely to be coupled therewith. Furthermore, in addition to the necessary vibration, unwanted spurious signals exist. The multiplier circuit 81 employs a method in which multiplication is achieved based on the main vibration possessed by the AT-cut crystal vibrator, that is, harmonics are generated and only the necessary harmonics are obtained as a harmonic signal. For this reason, unwanted spurious signals caused by the coupling of the main vibration and the sub-vibration of the AT-cut crystal vibrator and those vibrations become a factor for causing jitter to be generated. Furthermore, in the multiplier circuit 81, unwanted harmonics other than the high-frequency signal generated on the basis of the main vibration of the AT-cut crystal vibrator become a factor for causing jitter to be generated. Therefore, there is a problem in that those generation factors become noise, causing jitter to be increased.
The conventional voltage-controlled oscillators 1G and 1H are formed of the Colpitts-type oscillation circuit 80, the multiplier circuit 81, and the differential conversion circuit 82, and require many parts. Consequently, they are larger, presenting the problem of being contrary to the demand for ultra miniaturization and lower costs in recent years.
According to JP 8-213838, a configuration in which a plurality of SAW resonators are switched by a switch is employed. However, since control from an external source cannot be performed, there is a problem in that, once the SAW resonator is selected, a change after that cannot be performed.
Also, in the configuration shown in JP 8-213838, there is a problem in that, as shown in FIG. 22, the control voltage-oscillation frequency variable characteristics vary greatly due to the influence of (1) the temperature characteristics of a varicap diode, in particular, large variations in a low reverse voltage (a large capacitance value), (2) temperature-phase characteristic of active elements, and (3) temperature characteristics of passive elements such as coils and capacitors. That is, in the temperature characteristics of the oscillation frequency shown in FIG. 22(a), in particular, in a high temperature region, large variations in the oscillation frequency can be seen conspicuously. Similarly, also, in FIG. 22(b), large variations in the oscillation frequency can be seen in a high temperature region. These are problems which appear conspicuously in a voltage-controlled SAW oscillator (VCSO) using a SAW resonator.
In addition, when an oscillator is to be provided for each frequency used, there are problems in that design and manufacture become necessary for each model and, in order to respond instantly to an order from the system provider who requires this oscillator, the oscillator must be always in stock.
The present invention has been made to solve the above-described problems. An object of the present invention is to obtain a voltage-controlled oscillator capable of selectively causing several types of frequencies to oscillate under external control without using an LC parallel resonance circuit having a low Q value or an AT-cut crystal vibrator having unwanted sub-vibration and spurious signals in the oscillation section of the voltage-controlled oscillator. Another object of the present invention is to obtain a voltage-controlled oscillator whose size and cost are reduced and in which the amount of jitter is small without using a multiplier circuit and a differential conversion circuit. Another object of the present invention is to obtain a voltage-controlled oscillator in which the temperature characteristics of the oscillation frequency is improved, in particular, improvements are made in a high temperature region. Another object of the present invention is to obtain a voltage-controlled oscillator which can be used by making a selection from a plurality of oscillation frequencies without designing or manufacturing for each frequency.
In addition, another object of the present invention is to obtain an electronic device using a voltage-controlled oscillator which is capable of selecting a frequency according to the specification of the network system and in which the amount of jitter is small, and miniaturization and lower costs are achieved, for example, an optical network communication device.